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3PPs will tend to gravitate towards where there is most demand. They would still explore niche areas if consumers bought those particular products so as long as there is interest you wont lose that part of the 3pp product line.
As to your other point - well, my experience with Glorantha was that when Greg Stafford bowed out for a while (for whatever reason, probably commercial but I really don't know), the fans moved in and wrote loads of Gloranthan material. Then Greg came back and took up the reigns again, and everyone went with what Greg did because it was his world. The writers who found their material over-ruled got a bit fed up from time to time but it was just accepted that this was the risk that you took writing for a world you didn't own.
And I'm pretty sure Greg didn't feel that his hands were tied in any way by what other writers had produced.
I don't think that there would be very many fan-wars about what version of Golarion was the right one. Paizo's Golarion will always be the right one.